99 Yukon SLT fuel gauge is not reading correct. on 1999 GMC Yukon

I have gone about 240 miles and it is reading between 1/2 and 3/4 tank I know that's not correct. Then sometimes it will start to work. What would cause this?

Asked by for the 1999 GMC Yukon
Possible tank sender or gauge problem.... Scan test needed to confirm!.... NOT a code read at a parts store... A SCAN TEST! The scanner is capable of sweeping gauges to test!!!
Where could I get that scan test done? Also that 1/2 tank to 3/4 tank is on the full side.
Any good auto repair shop should be able to do it.
do you know what that scan would cost approx.?
I can't say, I'm in VA and prices vary, you may be able to just have the instrumentation tested and save a few bucks. Call around to find out! In any case it's cheaper than guessing...wrong!!